U.S. patent application number 14/783200 was filed with the patent office on 2016-02-18 for capacitive fill level sensor.
This patent application is currently assigned to BALLUFF GMBH. The applicant listed for this patent is BALLUFF GMBH. Invention is credited to Juergen GLOCK, Frank WINKENS.
Application Number | 20160047683 14/783200 |
Document ID | / |
Family ID | 51032867 |
Filed Date | 2016-02-18 |
United States Patent
Application |
20160047683 |
Kind Code |
A1 |
WINKENS; Frank ; et
al. |
February 18, 2016 |
CAPACITIVE FILL LEVEL SENSOR
Abstract
A capacitive fill level sensor for measuring the fill level of a
medium in a container has an electrode unit, which contains a
strip-shaped measurement electrode, a strip-shaped counter
electrode and a strip-shaped shielding electrode, the shielding
electrode at least partially surrounding the measurement electrode.
A first AC voltage source having a predefined frequency and
amplitude is provided, to which the shielding electrode is
connected such that a shielding capacitor formed between the
shielding electrode and the measurement electrode has a shielding
capacitance that is proportional to the length of the shielding
electrode. A second AC voltage source of equal frequency and a
predefined second amplitude is provided, the second amplitude being
in phase opposition to the first amplitude, to which AC voltage
source the counter electrode is connected, such that a measurement
capacitor formed between the counter electrode and the measurement
electrode has a measurement capacitance that is proportional to the
fill level. The measurement electrode voltage present at the
measurement electrode is used to determine the fill level.
Inventors: |
WINKENS; Frank;
(Ludwigshafen, DE) ; GLOCK; Juergen; (Hirschberg,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BALLUFF GMBH |
Neuhausen a.d.F. |
|
DE |
|
|
Assignee: |
BALLUFF GMBH
Neuhausen a.d.F.
DE
|
Family ID: |
51032867 |
Appl. No.: |
14/783200 |
Filed: |
April 8, 2014 |
PCT Filed: |
April 8, 2014 |
PCT NO: |
PCT/DE2014/100121 |
371 Date: |
October 8, 2015 |
Current U.S.
Class: |
73/304C |
Current CPC
Class: |
G01F 23/263 20130101;
G01F 23/266 20130101; G01F 23/268 20130101 |
International
Class: |
G01F 23/26 20060101
G01F023/26 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 2013 |
DE |
10 2013 005 963.1 |
Claims
1-17. (canceled)
18. Capacitive fill level sensor (10) for measuring the fill level
(H1, H2; H1', H2') of a medium (16) in a container (14), having an
electrode unit (12), which has a strip-shaped measurement electrode
(22), a strip-shaped counter electrode (24) and a strip-shaped
shielding electrode (26, 28, 40), wherein the shielding electrode
(26, 28, 40) surrounds the measurement electrode (22) at least
partially, wherein a first AC voltage source (60) having a
predefined frequency and amplitude is present, which is applied to
the ground (62) and to which the shielding electrode (26, 28, 40)
is connected such that a shielding capacitor (54, 56, 58) formed
between the shielding electrode (26, 28, 40) and the measurement
electrode (22) has a shielding capacitance that is proportional to
the length of the shielding electrode (26, 28, 40), wherein a
second AC voltage source (66) of the same frequency and with
predefined second amplitude is present, wherein the second
amplitude is in phase opposition to the first amplitude, which is
applied to the ground (62) and to which the counter electrode (24)
is connected such that a measurement capacitor (52) formed between
the counter electrode (24) and the measurement electrode (22) has a
measurement capacitance, which is proportional to the fill level
(H1, H2; H1', H2') and wherein the measurement electrode voltage
(72) applied to the measurement electrode (22) in relation to the
ground (62) is used to determine the fill level (H1, H2; H1',
H2').
19. Capacitive fill level sensor according to claim 18, wherein the
shielding electrode (26, 28, 40) has a third strip-shaped shielding
electrode (40), which is arranged on the rear side of the
measurement electrode (22) facing away from the container (14) and
covers the measurement electrode (22).
20. Capacitive fill level sensor according to claim 19, wherein the
shielding electrode (26, 28, 40), in addition to the third
shielding electrode (40), is also formed as a first strip-shaped
shielding electrode (26) and as a second strip-shaped shielding
electrode (28), wherein the first shielding electrode (26) is
arranged on the one side next to the measurement electrode (22) and
the second shielding electrode (28) is arranged on the other side
next to the measurement electrode (22) and wherein the first,
second and third shielding electrodes (26, 28, 40) are electrically
connected to one another.
21. Capacitive fill level sensor according to claim 20, wherein the
counter electrode (24), the measurement electrode (22) as well as
the first and second shielding electrode (26, 28) are arranged on a
carrier layer (48).
22. Capacitive fill level sensor according to claim 21, wherein an
insulation layer (50) is present at least in the region between the
third shielding electrode (40), on the one hand, and the first
shielding electrode (26), the measurement electrode (22) and the
second shielding electrode (28), on the other hand.
23. Capacitive fill level sensor according to claim 22, wherein the
insulation layer (50) is implemented as foam material adhesive
strip.
24. Capacitive fill level sensor according to claim 18, wherein the
rear side of the third shielding electrode (40) and the counter
electrode (24) is coated with a protective layer (42).
25. Capacitive fill level sensor according to claim 18, wherein the
electrode unit (12) has an adhesive layer (18) on the side facing
the container (14) to fix the electrode unit (12) on the outer wall
(20) of the container (14).
26. Capacitive fill level sensor according to claim 18, wherein the
measurement electrode (22), the counter electrode (24) as well as
the shielding electrode (26, 28, 40) are connected on a printed
circuit board (44) of a first electronic unit (30), which is
arranged directly on the container (14).
27. Capacitive fill level sensor according to claim 18, wherein a
plug connector (34) is present for connecting the electronic unit
(30).
28. Capacitive fill level sensor according to claim 18, wherein the
second AC voltage source (66) is implemented as an inverter, the
input of which is connected to the first AC voltage source
(60).
29. Capacitive fill level sensor according to claim 28, wherein the
inverter has a gain factor of one.
30. Capacitive fill level sensor according to claim 18, wherein the
frequency of the AC voltage sources (60, 66) is set to a value
between 0.1 MHz and 30 MHz.
31. Capacitive fill level sensor according to claim 30, wherein the
frequency is set to 1 MHz.
32. Capacitive fill level sensor according to claim 18, wherein a
rectifier (74) is provided for rectifying the measurement electrode
voltage (72) occurring at the measurement electrode (22) and
wherein the DC voltage (UDC) occurring at the output of the
rectifier (74) is used as an output signal (36), which is a
measurement for the fill level (H1, H2; H1', H2').
33. Capacitive fill level sensor according to claim 32, wherein an
impedance transformer (70) is present between the measurement
electrode (22) and the rectifier (74).
34. Capacitive fill level sensor according to claim 32, wherein the
first AC voltage source (60) is implemented as a controlled AC
voltage source (60), wherein the amplitude of the first AC voltage
source (64) is changeable as a function of a control voltage (UR),
wherein a comparator (84) is present, which compares the DC voltage
(UDC) with a reference voltage (URef) and sets the control voltage
(UR) as a function of the comparison result, whereby a control
circuit results, which maintains constant the measurement electrode
voltage (72) and wherein the control voltage (UR) is used as an
output signal (36), which is a measurement for the fill level (H1,
H2; H1', H2').
Description
[0001] The invention is based on a continuous capacitive fill level
sensor having an electrode unit according to the generic part of
the independent claim.
PRIOR ART
[0002] Capacitive fill level sensors can be used to measure fill
levels of fluid media or of solid materials. In the case of a
capacitive fill level sensor for measuring fill levels, a
measurement impedance is developed, the ohmic component of which,
but in particular the capacitive component of which, reflects a
measure for the fill level.
[0003] In a simple embodiment of an electrode unit, a measurement
electrode is provided, which is positioned, electrically insulated,
on the outer wall of a container or in an immersion probe adjacent
to a counter electrode.
[0004] In the published patent application DE 10 2009 017 011 A1, a
capacitive fill level sensor is described, which enables a
measurement of the fill height of a medium in a container. The
capacitive sensor has a measurement electrode and a counter
electrode, wherein the counter electrode is the electric ground,
which can correspond to the ground potential. The two electrodes
form a measurement capacitor having the medium as the dielectric.
The capacitance of the measurement capacitor depends on the fill
height of the medium. The capacitance of the measurement capacitor
is measured by means of a comparison with the capacitance of a
reference capacitor. Both capacitors are each connected to a
voltage source via a resistor. In order to carry out the
measurement, both capacitors are short-circuited by power switches
in temporal sequence and thus discharged. The voltage increase at
both capacitors following the opening of the switch depends on the
charging resistances and the capacitances. By means of an
assessment of the build-up time or by means of an assessment of the
temporal mean value of the voltages applied on the capacitors, the
fill height could be determined. In the exemplary embodiment shown,
however, the temporal mean values of the voltages are compared with
each other in a comparator. At the output of the comparator, a
switch signal is available, which signals that the fill height has
exceeded a certain measure or is below it.
[0005] In an exemplary embodiment, the measurement electrode is
surrounded with a shielding electrode on both sides and on the rear
side in order to eliminate the electromagnetic environmental
influences. The shielding is an active shielding, in the case of
which the potential of the shielding electrode is maintained on the
potential of the measurement electrode. The capacitance of the
capacitor, which is formed by the measurement electrode and the
shielding electrode, has a value of at least approximately
zero.
[0006] Due to an absolute measurement of the capacitance of the
measurement capacitor formed by the measurement electrode and the
counter electrode, the electrode unit is fixedly predefined and
must be calibrated in each case in view of the nature of the
medium.
[0007] A capacitive fill level sensor emerges from the published
patent application DE 199 49 985 A1, which is operated in the
context of an oscillation method. The operational frequency is in
the region of 5 to 10 MHz. In order to compensate the container
wall capacitance and to compensate an adhesive residue of the
electrically conductive medium in the region of the electrodes, a
further electrode is provided. The comparatively high operational
frequency up to 10 MHz puts correspondingly high requirements on
the electric shielding of the capacitive fill level sensor to meet
the EMC regulations. The switch arrangement to operate the
described measurement capacitor requires an absolute reference to
the ground potential. Due to this, the function of the previously
known capacitive fill level sensor depends on the design of the
container in which the medium is stored, the fill height of which
is to be measured.
[0008] The published patent application DE 10 2009 002 674 A1
describes a capacitive fill level sensor, in the case of which a
measurement electrode is provided, which forms the measurement
capacitor with an electric ground as the counter electrode. The
measurement capacitor is part of a series resonant circuit, the
resonant frequency of which depends on the impedance of the medium.
The conductibility of the medium has an influence on the quality of
the resonant circuit containing the measurement capacitor such that
the fill level of the medium can be determined by means of an
assessment of the amplitude and the frequency of the resonant
signal. By including the electric ground, the previously known
method can only be used in immersion probes having a grounded metal
housing, wherein the measurement electrode must always be arranged
close to the metal housing area.
[0009] In the published patent application DE 41 31 582 A1, a
capacitive fill level sensor is described, which has a measurement
electrode, a shielding electrode arranged behind the measurement
electrode and a counter electrode, wherein the counter electrode is
formed by the metallic container wall. The measurement electrode
and the metallic container wall form a measurement capacitor, the
capacitance of which depends on the fill level of the medium.
[0010] The utility model DE 7138801 U describes a capacitive fill
level sensor having an electrode unit immersed in the medium, said
electrode unit containing a strip-shaped measurement electrode and
a strip-shaped counter electrode. The two electrodes form a
measurement capacitor, the capacitance of which depends on the fill
level of the medium.
[0011] The measurement and counter electrodes are arranged adjacent
to each other at a dielectric container wall in contact with which
the medium is on one side. A shielding electrode is arranged on the
side of the measurement electrode facing away from the medium. The
measurement electrode and the shielding electrode are maintained on
the same potential, such that no electric field and thus no
capacitance can occur between the shielding electrode and the
measurement electrode. The measurement capacitor is thus formed
exclusively by the counter electrode and the measurement electrode,
wherein only the electric field, passing within the medium, is
effective, though not the electric field occurring between the
counter electrode and the shielding electrode. The measurement
result is thus not influenced by the capacitance developed between
the counter electrode and the shielding and thus depends at least
approximately only on the fill level of the medium.
[0012] The object underlying the invention is to specify a
capacitive fill level sensor which enables a simple adaptation to
different fill level measurement ranges or containers of different
heights.
[0013] The object is solved by the features specified in the
independent claim.
DISCLOSURE OF THE INVENTION
[0014] The invention is based on a capacitive fill level sensor for
the continuous measurement of the fill level of a medium in a
container, which has an electrode unit, which contains a
strip-shaped measurement electrode, a strip-shaped counter
electrode and a strip-shaped shielding electrode, wherein the
shielding electrode at least partially surrounds the measurement
electrode.
[0015] The capacitive fill level sensor according to the invention
is characterised in that a first AC voltage source having a
predefined frequency and amplitude is provided, to which the
shielding electrode is connected such that a shielding capacitor
formed between the shielding electrode and the measurement
electrode has a shielding capacitance that is proportional to the
length of the shielding electrode.
[0016] The capacitive fill level sensor according to the invention
is further characterised in that a second AC voltage source of the
same frequency and with predefined second amplitude is provided,
wherein the second amplitude is in phase opposition to the first
amplitude, to which the counter electrode is connected such that a
measurement capacitor formed between the counter electrode and the
measurement electrode has a measurement capacitance that is
proportional to the fill level.
[0017] The measurement electrode voltage which may be tapped at the
measurement electrode is dependent on the ratio of the shielding
capacitance to the measurement capacitance and is thus used to
determine the fill level. In this regard, the measurement electrode
voltage or a signal derived therefrom can be emitted as an output
signal for a measure of the fill level. Alternatively, the
measurement electrode voltage can be used in the context of a
control, wherein a control voltage can be provided as the output
signal for a measure of the fill level.
[0018] The capacitive fill level sensor according to the invention
is a highly flexible sensor for direct and continuous conversion of
the fill level of a medium in a container into a corresponding
output signal. As an output signal, an analogue voltage in the
range of 0 to 10 V, for example, or an impressed current in the
range of 4 to 20 mA, for example, can be provided.
[0019] The capacitive fill level sensor according to the invention
is preferably arranged on a non-metallic outer wall of the
container. The output signal reflects at all times a measure for
the actual height of the fill level in the entire measurement range
from zero, corresponding to the lower end of the electrode unit and
up to the maximum valve corresponding to the upper end of the
electrode unit.
[0020] A rather particular advantage of the capacitive fill level
sensor according to the invention having the electrode unit is that
the length of the electrode unit can be adapted individually by
simply_cutting to a predefined fill level measurement range,
corresponding to a predefined height of the container. The
capacitive fill level sensor according to the invention can thus be
manufactured and delivered inexpensively for example as bulk
goods.
[0021] The output signal, independently of the length of the
electrode unit, always uses the same electric range provided, which
is between the minimum and the maximum fill level to be measured,
wherein the only condition is that the thickness of the wall of the
container as well as, in particular, the electric properties of the
medium remain at least approximately the same. Thus a fill level
measurement range in the case of a container of, for example, 10 cm
maximum fill level or in the case of a high container of, for
example, 100 cm maximum fill level are distributed to the same
range of the output signal of 0 to 10 V or 4 to 20 mA already
mentioned by way of example without further engagement in a signal
processing arrangement.
[0022] Both the measurement capacitance of the measurement
capacitor and the shielding capacitance of the shielding capacitor
change equally with the freely selectable length of the electrode
unit and as a function of the fill level. Due to the
synchronisation of both capacitances, the fill level-dependent
proportion of the measurement capacitance in relation to the
shielding capacitance remains constant independently of the freely
configurable length of the electrode unit. Under the
above-mentioned condition, the output signal thus always passes
through the same hub or value range independently of whether the
length of the electrode unit is, for example only 10 cm or, for
example 100 cm.
[0023] Due to the phase opposition impact of the counter electrode,
on the one hand, and the shielding electrode, on the other hand,
with the AC voltages provided by both AC voltage sources, the
potential of the electric field lines is identical to the ground
potential or the ground in the geometric centre between the
measurement electrode and the counter electrode. The measurement
results are thus independent of the grounding conditions at the
container.
[0024] Advantageous developments and embodiments are each subject
matters of dependent claims.
[0025] A first embodiment makes provision for the shielding
electrode to be designed as a third strip-shaped shielding
electrode, which is arranged on the rear side of the measurement
electrode facing away from the container and for the third
shielding electrode to cover the measurement electrode. With this
measure, not only the shielding capacitor is formed, but an
electromagnetic shielding is also simultaneously achieved against
disturbance signals from the environment.
[0026] An alternative or additional embodiment makes provision for
the shielding electrode to be designed as a first strip-shaped
shielding electrode and as a second strip-shaped shielding
electrode, for the first shielding electrode to be arranged on the
one side next to the measurement electrode and the second shielding
electrode to be arranged on the other side next to the measurement
electrode and for the first, second and third shielding electrodes
to be electrically connected to one another. Due to the fact that
the first and second shielding electrodes are arranged in the same
plane as the measurement electrode, a simple installation of the
first and second shielding electrode results.
[0027] The wall of the container is located in the electric field
between the first shielding electrode and the measurement electrode
or between the second shielding electrode and the measurement
electrode. The value of the two partial shielding capacitances is
thus dependent on the dielectric of the wall of the container. With
an increase of the dielectric of the wall of the container, not
only the shielding capacitance increases, but also the coupling or
the voltage at the measurement electrode resulting from the
coupling. However, the coupling of the measurement electrode to the
medium also increases simultaneously. The influence of the material
of the wall of the container is compensated in this way within
certain limits. The same applies for an adhesive residue of foaming
media adhering to the inner wall of the container, said media may
occur in particular in the case of a decreasing fill level.
[0028] A development of this embodiment makes provision for the
counter electrode, the measurement electrode as well as the first
and second shielding electrode to be arranged on a carrier layer,
which is implemented, for example, as a flexible printed circuit
board.
[0029] According to one embodiment, provision is made for an
insulation layer to be provided at least in the region between the
third shielding electrode, on the one hand, and the measurement
electrode, the first shielding electrode as well as the second
shielding electrode. The insulation layer, which preferably has a
low dielectric constant, enables a simple specification of the
shielding capacitance, during manufacturing, in relation to the
unit of length of the electrode unit.
[0030] The insulation layer is preferably implemented as a foam
material adhesive tape. A simple adaption of the electrode unit to
the curve of the outer wall of the container is thus, in
particular, possible.
[0031] One embodiment makes provision for the rear side of the
electrode unit, corresponding to the rear side of the third
shielding electrode and the counter electrode, to be coated with an
insulating protective layer. The electrodes manufactured from, for
example, copper foil are thus protected against environmental
influences.
[0032] Another embodiment makes provision for an adhesive layer to
be provided on the side of the electrode unit facing the container
to fix the electrode unit on the outer wall of the container. The
adhesive layer enables, in particular a simple installation on a
curved outer wall of the container.
[0033] Another development of the capacitive fill level sensor
according to the invention makes provision for the measurement
electrode, the counter electrode as well as the shielding electrode
to be directly connected on a printed circuit board of a first
electronic unit, which is arranged directly on the container. The
electrodes are directly soldered on the printed circuit board. In
particular, the electronic unit can contain a signal processing
arrangement for controlling the electrodes as well as the complete
evaluation circuit, such that an output signal can be provided at
the output of the first electronic unit, which reflects the fill
level.
[0034] Alternatively, a second electronic unit separated from the
electrode unit can be provided.
[0035] One embodiment of the capacitive fill level sensor according
to the invention makes provision for the second AC voltage source
to be implemented as an inverter, the input of which is connected
to the first AC voltage source. With this measure, the
implementation of the second AC voltage source is particularly
inexpensive, wherein the provision of the phase opposition AC
voltage is simultaneously ensured. The inverter is preferably set
to a gain factor of at least approximately one. By changing the
gain factor, an adaption to different geometries of the electrodes
can take place without particular effort.
[0036] Another embodiment makes provision for the frequency of the
AC voltage sources to be set to a value between 0.1 MHz and 30 MHz.
The selection of the frequencies in the indicated range enables, on
the one hand, a sufficient coupling of the AC voltage from the
shielding electrode and the counter electrode to the measurement
electrode. On the other hand, the AC voltages in this frequency
range can be implemented with simple means. The frequency is
preferably, for example, set to at least approximately 1 MHz.
[0037] One embodiment makes provision for a rectifier to rectify
the measurement electrode voltage occurring at the measurement
electrode, wherein the DC voltage applied to the output of the
rectifier can be used as an output signal, which can be assessed as
a measure for the fill level.
[0038] Due to the anticipated low capacitances and thus high source
impedance of the capacitive fill level sensor according to the
invention, an impedance transformer is preferably connected between
the measurement electrode and the rectifier, said impedance
transformer only slightly charging the measurement electrode and
being able to control the downstream rectifier at low
resistance.
[0039] A particularly advantageous development makes provision for
the first AC voltage source to be implemented as a controlled AC
voltage source, the output voltage of which is changeable as a
function of a control voltage.
[0040] The control voltage is set as a function of the output
signal of a comparator, which compares the DC voltage provided by
the rectifier to a fixedly predefined reference voltage. A closed
control circuit thus results, which keeps the measurement electrode
voltage which can be tapped at the measurement electrode constant.
In the case of this development, the control voltage can be used as
output voltage, which reflects a measure for the fill level.
Ultimately, in the case of this development, the measurement
electrode voltage which can be tapped at the measurement electrode
is also used to determine a measure for the fill level, although
the measurement electrode voltage is maintained constant.
[0041] Further advantageous developments and embodiments of the
capacitive fill level sensor according to the invention result from
the following description.
[0042] Exemplary embodiments of the invention are depicted in the
drawing and explained further in the following description.
BRIEF DESCRIPTION OF THE FIGURES
[0043] FIG. 1 shows a capacitive fill level sensor according to the
invention, which is cut in the region of an electrode unit in the
vertical direction (dt.: Hohenrichtung),
[0044] FIG. 2 shows a section in the vertical direction through a
capacitive fill level sensor according to the invention,
[0045] FIG. 3 shows a cross-section through an electrode unit of a
capacitive fill level sensor according to the invention,
[0046] FIG. 4 shows a first exemplary embodiment of a signal
processing arrangement,
[0047] FIG. 5 shows a functional connection between an output
signal of the signal processing arrangement shown in FIG. 4 and
fill levels.
[0048] FIG. 6 shows a second exemplary embodiment of a signal
processing arrangement and
[0049] FIG. 7 shows functional connections between an output signal
of the signal processing arrangement shown in FIG. 6 and fill
levels.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0050] FIG. 1 shows a capacitive fill level sensor 10 according to
the invention, which is cut in the region of an electrode unit 12
in the vertical direction. The capacitive fill level sensor 10
measures the fill level H1, H2 of a medium 16 located in a
container 14 in a fill level measurement range H. In the exemplary
embodiment shown, the medium 16 has a first fill level H1. A
possible second fill level H2 is also displayed.
[0051] The electrode unit 12 is adhered to the outer wall 20 of the
container 14 by means of an adhesive layer 18. Due to the partially
cut depiction, a measurement electrode 22, a counter electrode 24,
a first shielding electrode 26 as well as a second shielding
electrode 28 are visible. The electrode unit 12 is connected to a
first electronic unit 30, which is arranged at the lower end of the
container 14 in the exemplary embodiment shown. An output signal 36
is provided via a line 32 which is contacted by means of a plug
connector 34 with the first electronic unit 30, said output signal
being a measure for the fill level H1, H2 or all occurring fill
levels in the fill level measurement range H of the medium 16 in
the container 14.
[0052] The capacitive fill level sensor 10 according to the
invention shown in FIG. 2 and cut in the region of the measurement
electrode 22 in the vertical direction shows a third shielding
electrode 40 arranged on the rear side of the measurement electrode
22.
[0053] The parts shown in FIG. 2, which match the parts shown in
FIG. 1, each bear the same reference numbers. This also applies for
the following figures.
[0054] The electrode unit 12 is at least on the rear side
surrounded by a protective layer 42. The electrodes 22, 24, 26, 28,
40 are contacted with a printed circuit board 44 arranged in the
first electronic unit 30, for example by means of soldering. The
first electronic unit 30 contains a signal processing arrangement
46.
[0055] FIG. 3 shows a cross-section through the electrode unit 12
of the capacitive fill level sensor 10 according to the invention.
The container wall 20 as well as the electrode unit 12 are shown
linearly such that the capacitive fill level sensor 10 according to
the invention is, for example, positioned on a rectangular
container 14. In the case of a cylindrical container 14, the outer
wall 20 has a curve, to which the electrode unit 12 can be readily
adapted due to the construction thereof described in detail
below.
[0056] The electrode unit 12 is adhered to the non-metallic outer
wall 20 of the container 14 by means of the adhesive layer 18. The
adhesive layer 18 is applied on the side of a carrier layer 48
facing the container 14, preferably on a flexible printed circuit
board, which preferably contains the counter electrode 24, the
first shielding electrode 26, the measurement electrode 22 as well
as the second shielding electrode 28 as conductor paths.
[0057] An insulation layer 50 is provided on the rear side of the
measurement electrode 22 as well as the first and second shielding
electrode 26, 28, said insulation layer preferably having a low
dielectric constant. The insulation layer 50 is, for example,
produced from a foam material adhesive tape. The insulation layer
50 separates the measurement electrode 22 as well as the first and
second shielding electrode 26, 28 from the third shielding
electrode 40.
[0058] The rear region of the electrode unit 12 in relation to the
container 14 is surrounded by a protective layer 42, which protects
the electrode unit 12, in particular from environmental
influences.
[0059] The complete electrode unit 12 is manufactured from flexible
materials such that the electrode unit 12 can be readily adapted to
different outer wall curves of cylindrical or oval containers
14.
[0060] In FIG. 3, the individual components of the electrode unit
12 are depicted significantly enlarged to illustrate the
construction. The following dimensions can be provided by way of
example: the widths of the measurement electrode 22 as well as of
the first and second shielding electrode 26, 28 can be, for
example, 3.5 mm, while the width of the third shielding electrode
40 can be, for example between 8 to 13 mm. The width of the counter
electrode 24 is, for example, 8 mm. The thickness of the insulation
layer 50 is, for example, 1 mm. The thicknesses of the adhesive
layer 18, the electrodes 22, 24, 26, 28, 40 as well as the
protective layer 42 are in the micrometre range. The thickness of
the carrier material 48, which is, for example, implemented as a
flexible printed circuit board, is, for example, in the upper
micrometre range.
[0061] A significant advantage of the construction of the
capacitive fill level sensor 10 according to the invention having
the electrode unit 12 shown is that the electrode unit 12 can be
adapted by the user in a simple manner to different fill level
measurement ranges H corresponding to different heights of
containers 14 by shortening the electrode unit 12, for example by
means of scissors, to the required length. The capacitive fill
level sensor 10 according to the invention can thus be manufactured
and provided, for example, as bulk goods.
[0062] The electrodes 22, 26, 28, 40, to an extent, form a
half-coaxial structure, in the case of which the measurement
electrode 22 is comparable with the inner conductor of a coaxial
line, which is open to the outer wall 20 of the container 14, yet
is shielded at the sides by the first and second shielding
electrode 26, 28 and at the rear by the third shielding electrode
40.
[0063] In the exemplary embodiment shown in FIGS. 1 and 2, the
first electronic unit 30 is positioned at the lower end of the
electrode unit 12. In another embodiment, not shown in further
detail, the first electronic unit 30 can be installed at any
vertical position of the electrode unit 12 and can be contacted
with the electrode unit 12.
[0064] In a further embodiment, a second electronic unit 12, not
shown in further detail, is provided instead of the first
electronic unit 30, which is directly connected to the electrode
unit 12, said second electronic unit being arranged separated from
the electronic unit 12. In this case, the electrode unit 12 is
connected to the second electronic unit with an at least S-wire,
preferably pluggable cable.
[0065] The printed circuit board 44, the ends of the electrodes 22,
24, 26, 28, 40 lying inside a housing of the first electronic unit
30, the signal processing arrangement 46 as well as the further
components of the first electronic unit 30, can be surrounded with
a fill material, for example casting resin, such that the first
electronic unit 30 is protected, in particular against
environmental influences.
[0066] Due to the construction of the electrode unit 12, a
measurement capacitor 52 is formed between the measurement
electrode 22 and the counter electrode 24, said measurement
capacitor having a fill level-dependent measurement capacitance.
The measurement capacitance has a small basic amount, which is
linearly dependent on the fill level measurement range H. The
measurement capacitance, in particular, however, has a fill
level-dependent value that is proportional to fill level H1, H2 of
the medium 16 in the container 14.
[0067] A first shielding capacitor 54 is formed between the first
shielding electrode 26 and the measurement electrode 22; a second
shielding capacitor 56 is formed between the measurement electrode
22 and the second shielding electrode 28 and a third shielding
capacitor 58 is formed between the measurement electrode 22 and the
third shielding electrode 40. The capacitances of the shielding
capacitors 54, 56, 58 are exclusively dependent and proportional to
the fill level measurement range H, corresponding to the length of
the electrode unit 12. The shielding capacitances thus increase
linearly with the length of the electrode unit 12.
[0068] In FIG. 4, an exemplary embodiment of the signal processing
arrangement 46 is shown, which is provided for operating the
electrode unit 12.
[0069] The signal processing arrangement 46 contains a first AC
voltage source 60, which is connected between a ground 62 and the
shielding electrodes 26, 28, 40 electrically connected to one
another, corresponding to the shielding capacitors 54, 56, 58. The
first AC voltage source 60 provides a first, preferably sinusoidal
AC voltage 64, the frequency of which is, for example in the range
of 0.1 to 30 MHz. The frequency of the first AC voltage is
preferably set to 1 MHz. The frequency is to be set in such a way
that, on the one hand, only a small undesired emission of the
signal takes place and, on the other hand, however, a sufficiently
high signal level occurs at the measurement electrode 22 in view of
the comparatively low capacitances occurring, which are in the
picofarad range.
[0070] Furthermore, a second AC voltage source 66 is provided,
which is implemented in the exemplary embodiment shown as an
inverter. The second AC voltage source 66 is connected to the
counter electrode 24. The second AC voltage source 66 provides a
second AC voltage 68, which has the same frequency as the first AC
voltage 64, which, however, is phase-shifted by 180.degree., i.e.
is in phase opposition to the first AC voltage 64.
[0071] If necessary, there is a comparison possibility for the
amplitude of the first or the second AC voltage 64, 68, in order to
be able to adapt at least one AC voltage 64, 68 to different
geometries of the electrodes 22, 24, 26, 28, 40. In the exemplary
embodiment shown, it is assumed that the second AC voltage source
66 implemented as an inverter has the gain factor 1, such that the
amplitude of the first AC voltage 64 is at least approximately
equal to the amplitude of the second AC voltage 68.
[0072] The measurement electrode 22 is preferably connected to an
impedance transformer 70, which only slightly charges the
measurement electrode 22, yet passes on a measurement electrode
voltage 72 occurring at the measurement electrode 22 to a
downstream rectifier 74 at low resistance. The rectifier 74
provides a DC voltage UDC, which corresponds to the rectified
measurement electrode voltage 72.
[0073] The measurement capacitor 52, on the one hand, and the
shielding capacitors 54, 56, 58 lying parallel, on the other hand,
form a capacitive voltage divider. A divided, fill level-dependent
measurement electrode voltage 72 occurs at the measurement
electrode 22. The sum of the shielding capacitances of the
shielding capacitors 54, 56, 58 forms the reference.
[0074] With increasing fill level of the medium 16, the capacitance
of the measurement capacitor 52 increases with respect to the
constant shielding capacitance of the shielding capacitors 54, 56,
58. The measurement electrode voltage 72 decreases in the event of
rising fill level H1, H2 of the medium 16 because the voltages
behave contrarily to the capacitances of the capacitors 52, 54, 56,
58.
[0075] In FIG. 4, the first AC voltage 64 as well as the second AC
voltage 68 are each recorded with constant amplitude and the
measurement electrode voltage 72 with two different amplitudes,
wherein in the case of a smaller fill level H1, the higher
amplitude (continuous line) occurs and in the case of a higher fill
level H2, the lower amplitude (dotted line) occurs.
[0076] The measurement electrode voltage 72 could already be used
directly as a measure for the fill level H1, H2, wherein the
highest measurement electrode voltage 72 occurs in the case of the
lowest measurable fill level H1, H2 and the lowest measurement
electrode voltage 72 in the case of the highest measurable fill
level H1, H2. However, in the case of this exemplary embodiment,
the DC voltage UDC is preferably used instead of the measurement
electrode voltage 72 directly as the measure for the fill level H1,
H2 and is provided as the output signal 36. In the case of this
exemplary embodiment of the signal processing arrangement 46, the
variable measurement electrode voltage 72 is used as a measure for
determining the fill level H1, H2 of a medium 16 in a container
14.
[0077] FIG. 5 shows functional connections between the DC voltage
UDC for two different fill height measurement ranges H, H', which
are provided for two containers 14 of different height. The DC
voltage UDC corresponds to the output voltage 36 of the capacitive
fill level sensor 10.
[0078] By means of further functional blocks, not shown, the DC
voltage UDC can be converted into a predefined range of the output
signal 36. For example, the output signal 36 can be converted and
output in the range of 0 to 10 V or the range of 4 to 20 mA.
[0079] In the exemplary embodiment shown, the smallest DC voltage
UDC corresponding to the higher fill level H2 is not set to the
value zero. The smallest value of the DC voltage UDC can of course
be set to the value zero as a function of the desired design.
[0080] The significant advantage of the capacitive fill level
sensor 10 according to the invention is that the electrode unit 12
can be adapted by the user himself to the required fill level
measurement range H, H' by simply shortening the longer electrode
unit 12 delivered.
[0081] The functional connections shown in FIG. 5 immediately allow
a further advantage of the capacitive fill level sensor 10
according to the invention be recognised, which is that without
further action by the user, any capacitive fill level sensor 10 cut
to size provides the same voltage range of the DC voltage UDC or
the same voltage range of the output signal 36 as a function of the
respective fill level measurement range H, H'. The scaling for
different fill level measurement ranges H, H' is independent of the
length of the electrode unit 12. The smaller fill level measurement
range H having the fill levels H1, H2 shown by way of example uses
the entire available voltage range of the DC voltage UDC or the
output signal 36 just like the larger fill level measurement range
H' having the fill levels H1', H2' shown by way of example. In the
case of at least approximately identical media 16, no engagement in
the signal processing arrangement 46 is required for this
purpose.
[0082] FIG. 6 shows another exemplary embodiment of the signal
processing arrangement 46, which provides an output signal 36 that
is proportional to the fill level H1, H2 of the medium 16, i.e. the
output signal 36 similarly increases with increasing fill level H1,
H2.
[0083] The first AC voltage source 60 is designed in this exemplary
embodiment as a controllable first AC voltage source 60, wherein
the amplitude of the first AC voltage 64 is changeable by means of
a control voltage UR. The amplitude of the first AC voltage 64 is
thus predefined by the control voltage UR. In the case of this
exemplary embodiment, the DC voltage UDC is made available to a
comparator 84, which compares the DC voltage UDC with a reference
voltage URef provided by a reference voltage source 86 and provides
the control voltage UR as a function of the comparison result.
[0084] The reference voltage URef is, for example 1 V. The
comparator 84 is, for example implemented as a high-gain
differential amplifier such that the output signal is proportional
to the difference between the DC voltage UDC and the reference
voltage URef. If required, a comparing element can also be used as
the comparator 84. In this case, it must be ensured that the
resulting control circuit is sufficiently damped in order to avoid
control oscillations.
[0085] The resulting control circuit ensures that the first AC
voltage 64 and thus the second AC voltage 68 are controlled to an
amplitude at which the measurement electrode voltage 72 and
correspondingly the DC voltage UDC resulting therefrom can be
maintained constant and at the value of the reference voltage URef.
In FIG. 6, the first AC voltage 64 as well as the second AC voltage
68 are thus depicted with a high amplitude (continuous line)
corresponding to a higher fill level H2 and with a lower amplitude
(dotted line) corresponding to a lower fill level H1, while the
measurement electrode voltage 72 is depicted as constant.
[0086] In the case of this exemplary embodiment of the signal
processing arrangement 46, the control voltage UR can also be used
as the output signal 36, which is proportional to the fill level
H1, H2; H1', H2' of the medium 16 in the container 14. Also in the
case of this exemplary embodiment of the signal processing
arrangement 46, the measurement electrode voltage 72, maintained
constant in this exemplary embodiment, is ultimately used as a
measure for determining the fill level H1, H2; H1', H2' of a medium
16 in a container 14.
[0087] In FIG. 7, two functional connections between the control
voltage UR and the fill levels H1, H2; H1', H2' are shown for two
different fill level measurement ranges H, H'. The control voltage
UR corresponds to the output voltage 36 of the capacitive fill
level sensor 10.
[0088] Also in the case of this exemplary embodiment of the signal
processing arrangement 46 according to the invention, the output
signal 36 of course scales to the entire predefined range of, for
example 0 to 10 V or for example 4 to 20 mA without engagement in
the signal processing arrangement 46 by the user, independently of
the cut length of the electrode unit 12 and thus independently of
the set fill level measurement range H, H'.
[0089] It is visible from FIG. 7 that a low level of the control
voltage UR or of the output signal 36 corresponds to a low fill
level H1, H1' and a higher level of the control voltage UR or of
the output signal 36 corresponds to a higher fill level H2, H2'.
Two different fill levels H1, H2; H1', H2' are again recorded by
way of example, which can occur for two different fill level ranges
H, H'.
* * * * *